Fibroblast Growth Factor 21 (FGF21) is a secreted polypeptide that belongs to a subfamily of Fibroblast Growth Factors (FGFs) that includes FGF19, FGF21, and FGF23 (Itoh et al., 2004, Trend Genet. 20: 563-69). FGF21 is an atypical FGF in that it is heparin independent and functions as a hormone in the regulation of glucose, lipid, and energy metabolism.
FGF21 is highly expressed in liver and pancreas and is the only member of the FGF family to be primarily expressed in liver. Transgenic mice overexpressing FGF21 exhibit metabolic phenotypes of slow growth rate, low plasma glucose and triglyceride levels, and an absence of age-associated type 2 diabetes, islet hyperplasia, and obesity. Pharmacological administration of recombinant FGF21 protein in rodent and primate models results in normalized levels of plasma glucose, reduced triglyceride and cholesterol levels, and improved glucose tolerance and insulin sensitivity. In addition, FGF21 reduces body weight and body fat by increasing energy expenditure, physical activity, and metabolic rate. (Xu et al. 2009 Diabetes 58:250-259) There is also evidence that FGF21 may have a protective effect in acute pancreatitis (Johnson et al. 2009 Gasteroenterology 137(5):1795-1804).
Experimental research provides support for the pharmacological administration of FGF21 for the treatment of type 2 diabetes, obesity, dyslipidemia, and other metabolic conditions or disorders in humans. (Xu et al., 2009 Am J Physiol Endocrinol Metab. 297:E1105-E1114; Reviewed in Ryden 2009 Cell Mol Life Sci. 66:2067-73)
FGF21 is a liver derived endocrine hormone that stimulates glucose uptake in adipocytes and lipid homeostasis through the activation of its receptor. Interestingly, in addition to the canonical FGF receptor, FGFR1c, the FGF21 receptor also consists of the membrane associated β-Klotho as an essential cofactor. Activation of the FGF21 receptor leads to multiple beneficial effects on a variety of metabolic parameters.
In mammals, FGFs mediate their action via a set of four FGF receptors FGFR1-4 that in turn are expressed in multiple spliced variants. Each FGF receptor contains an intracellular tyrosine kinase domain that is activated upon ligand binding, leading to downstream signaling pathways involving MAPKs (Erk1/2), RAF1, AKT1 and STATs. (Kharitonenkov et al., 2008 BioDrugs 22:37-44). Several reports suggested that the “c”-reporter splice variants of FGFR1-3 exhibit specific affinity to βKlotho and could act as endogenous receptor for FGF21 (Kurosu et al., 2007 J. Biol. Chem. 282:26687-26695); Ogawa et al., 2007 Proc. Natl. Acad. Sci. USA 104:7432-7437; Kharitonenkov et al., 2008 J. Cell Physiol. 215, 1-7). In the liver, which abundantly expresses both βKlotho and FGFR4, FGF21 does not induce phosphorylation of MAPK albeit the strong binding of FGF21 to the βKlotho-FGFR4 complex. In 3T3-L1 cells and white adipose tissue, FGFR1 is by far the most abundant receptor, and it is therefore most likely that FGF21's main functional receptors in this tissue are the βKlotho-FGFR1c complexes.
The present disclosure provides binding proteins, including bispecific binding proteins, that can activate FGF21-mediated signaling, i.e., an agonistic binding protein that mimics the function of FGF21. Such a binding protein can be a molecule with FGF21-like activity and selectivity but with added therapeutically desirable characteristics such as protein stability, low immunogenicity, ease of production and a desirable in vivo half-life.